Fibreglass is a very durable material that is used for a variety of different things. Many large parts that are built with fibreglass are made up of multiple sections that are welded together to make a whole. The process of welding fibreglass is used to join fibreglass together or repair damaged fibreglass. Once fibreglass is applied on top of another piece of fibreglass, or over a seam, it becomes one solid piece of fibreglass. All fibreglass supplies that will be used for welding fibreglass can be found at your local marine supply shop.
Connect the pieces of fibreglass that you are joining together and hot glue the seam. Put on a respirator and lightly grind the seam and 20 cm (8 inches) on each side of the seam using a grinder. Blow off all the dust and prop the piece up so the entire seam is accessible.
Determine how thick the walls of the fibreglass you are connecting is and make your weld the same thickness using fibreglass mat. Make every two layers 5 cm (2 inches) bigger than the last so the edges of the weld tapers off and doesn't form ledges when it hardens. If the seam is large make each weld no longer than 60 cm (2 feet), any longer and it will be harder to work with.
Put on a pair of rubber gloves and mix a small bucket of polyester resin with two per cent catalyst. Thoroughly mix the two together using a paint stir. Lay a piece of scrap cardboard on a table that is at least 30 cm (1 foot) longer and wider than your welds are. Dip a 10 cm (4 inch) felt roller into the resin and roll it onto the cardboard to wet it out. Lay the widest layer of mat onto the cardboard and saturate the layer of mat with resin using the felt roller. Continue the same process adding the layers from largest to smallest keeping each layer centred until you have all the layers sandwiched into one weld.
Pick the weld up from the cardboard and carefully lay it onto the seam with the seam perfectly centred under the weld. Roll the entire weld with an air roller, which is a metal hand held roller that can be found anywhere fibreglass supplies are sold, to remove all the trapped air and flatten the weld out. Add the rest of the welds following the same process until the entire seam is welded with fibreglass. Let the fibreglass harden for a few hours and then sand the weld smooth using 100 grit sandpaper.
Keep the piece still and secured while the weld is hardening because the heat from the weld will soften the glue slightly.
Always wear a respirator when working with fibreglass.